Presently, optical fibers for use in commonly popular optical fiber communication systems have a structure in which the outer circumferential surface of a core is surrounded by a cladding. Optical signals are propagated through this core to transmit information. In these years, with the wide spread use of optical fiber communication systems, the volume of information to be transmitted is dramatically increased. In order to achieve an increase in the transmission capacity of these optical fiber communication systems, it is known that a multicore fiber having a plurality of cores and a cladding surrounding the outer circumferential surfaces of the plurality of cores is used to transmit a plurality of signals with light beams propagating through the cores.
For a coherent optical communication line to achieve an increase in the transmission capacity, a polarization-maintaining fiber is known. The polarization-maintaining fiber is an optical fiber that can propagate light beams with the polarized state of the light beams being maintained by giving a difference between the propagation constants of polarized waves orthogonal to each other. For example, for such polarization-maintaining fibers, there are an elliptical core polarization-maintaining fiber having an elliptical core in a cross section (see Patent Literature 1 below), a stress-applying polarization-maintaining fiber (in the following, referred to as a “PANDA” polarization-maintaining fiber) in which a core is sandwiched between a pair of stress applying parts to apply stress to the core for forming a core having orthogonal axes with the effective refractive indexes different from each other (see Patent Literature 2 below), and other polarization-maintaining fibers.    [Patent Literature 1] JP-A-2013-80126    [Patent Literature 2] JP-A-362-178909